There are a number of known techniques for constructing polygon mirrors in the prior arts. Generally, the larger the number of rotations, the more capable the polygon mirror, so it is driven to rotate at a high speed of, for example, 30,000 rpm. However, when a polygon mirror, i.e., a polygon rotation body having mirror surfaces rotates as such a high speed, it is deformed by centrifugal force. If such deformation is not uniform, irregularities occur in mirror surfaces, resulting in a disturbance of images. Also when rotating at high speed, a so-called precession motion and deflections are also liable to occur. When a polygon mirror moves in the form of the precession motion, naturally the image is disturbed. For this reason, various kinds of supporting devices for polygon mirrors have been proposed.
For example, in the art disclosed in Japanese Patent Public Disclosure No. 59-28757, a metal is used in sliding surfaces of a rotor, and the rotor is structured to receive a radial load by a dynamic pressure generated by grooves which is arranged in a herringbone configuration. However, when a radial load is increased at the time of high-speed rotation, the ordinary task of supporting the polygon mirror using an air film made by dynamic pressure generation grooves of a herringbone configuration becomes difficult. Balancing adjustments are also troublesome and neither can precession motion be prevented.
The present applicant proposes a polygon mirror capable of rotating stably at high speed and reflecting laser beams with a high degree of precision and which was disclosed in Japanese Patent Public Disclosure No. 63-241515.
For further understanding of the present invention, the prior art in the above-mentioned Japanese Patent Public Disclosure No. 63-241515 will now be explained with reference to FIG. 10.
In a laser printer, for example, laser beams from a laser unit comprising a semiconductor laser or gas laser pass through a window 13 and are reflected by mirrors 2 formed on the circumferential surfce of a rotor 3, and then directed to the surface of a sensitizing body. A magnet member 7 is integrally mounted in the rotor 3 with a backup ring 9. The radial load of rotor 3 is born by a stationary shaft 5 provided at its center and its own-weight is received by a thrust plate 10. Though not shown, a prior art in which a thrust plate is provided on the upper part of the rotor 3 is known, too. Grooves 11 of a herringbone configuration and a spiral configuration are formed on the sliding surface of the stationary shaft 5 and the thrust plate 10. Dynamic pressure is generated by an air film and the radial load and the thrust load are thereby sustained.
The stationary shaft 5 is fixedly mounted on a casing 4 and on this casing are provided stator coils 6 facing opposite to the magnet member 7.
In such a prior art, the mirrors 2 are convered with evaporation of aluminum or the like, so it is difficult to achieve a high degree of accuracy (ca. several microns). Since the magnet member 7 is buried discontinuously in a rotor, ununiform deformation is apt to be caused in such a rotor, particularly when rotated at high-speed. The mirror surfaces of a rotor are therefore deformed, resulting in a disturbance of the motion of reflected light.
There is a known method in which a polygon rotor is made by employing an aluminum alloy as mirror surfaces. In this case, however, the rotor is fragile and liable to be deformed.
As a result of various studies, the present inventors found that though ununiform deformations of a rotor are undesirable, no problems arise when a rotor is uniformly deformed by a centrifugal force, and that by improving construction of transmission path for rotational torque given to a magnet member, ununiform deformations can be prevented.
Therefore, an object of the present invention is to provide a rotation supporting device of a polygon mirror whereby ununiform deformations in a rotor are prevented.